The present invention generally relates to an optical system employing prisms and an optical information processing system incorporating such optical system. More particularly, the invention concerns an optical system employing prisms which is suited for modifying or changing the shape of a light beam having a two-dimensional distribution as well as an optical information processing apparatus in which the optical system employing prisms are used.
As the optical system for modifying or varying or converting the cross-sectional shape of a light beam, there have hithertofore been known two types of optical systems, i.e. an optical system in which a pair of cylindrical lenses are disposed confocally, wherein magnification power of the optical system is adjusted in dependence on the ratio of the focal lengths of the cylindrical lenses (e.g. U.S. Pat. No. 4,203,652), and an optical system in which a triangle prism is used and the difference existing between the angle of incidence and the output angle of the light beam due to refraction of the prism is made use of for conversion of the shape of the light beam (e.g. U.S. Pat. No. 4,333,173). The present invention concerns an improvement of the last mentioned type optical system.
FIG. 1 of the accompanying drawings illustrates refraction of a light beam by a triangle prism, in which a light beam is incident on one face of a triangle prism along a direction inclined relative to the one face. Referring to the figure, when the incident angle (i.e. angle of incidence) and the output angle at the boundary between air and the prism media are represented by .theta..sub.1 and .theta..sub.2, respectively, the following relation applies valid (in accordance with Snell's law). EQU sin .theta..sub.1 =n.multidot.sin .theta..sub.2 ( 1)
where n represents the refractive index of the medium or material constituting the triangle prism.
Further, magnification ratio M of the beam diameter brought about by the refraction is given by EQU M=D.sub.2 /D.sub.1 =cos .theta..sub.2 /cos .theta..sub.1 ( 2)
where D.sub.1 and D.sub.2 represent the diameters of the incident beam and the output beam, respectively. By making use of the relation mentioned above, modification or conversion of the beam shape can be realized. On another prism face, opposite to the inclined one, the light beam is incident or outputted in a direction perpendicular to said other prism face, as seen in FIG. 1, so that the ratio of the beam diameter generated at the inclined face undergoes no change at the face opposite thereto. In case the beam leaves the prism in the direction perpendicular to the output face thereof, as described above, there apply valid among the incident angle .theta..sub.1, apex angle .phi., refractive index n and the magnification ratio of beam diameter M the following relations: ##EQU1## provided that M.gtoreq.1.
In this connection, it is noted that when the magnification ratio M of the beam diameter is excessively large (e.g. about 3 or more), the reflection factor of the prism at the incident face will become too large to be useful in practical applications. Under the circumstance, with the aim of realizing a greater magnification ratio of beam diameter while evading the above problem, an attempt has been proposed in which a plurality of prisms made of a material exhibiting the same refractive index are used to constitute a prism system. By way of example, FIG. 2 of the accompanying drawings shows an optical system in which a pair of prisms 1 and 2 are employed.
The relations mentioned above are invariable so far as the wavelength of the light beam remains constant. However, when the wavelength is varied, the refractive index of the medium constituting the triangle prism undergoes variation or change, bringing about a corresponding change in the output angle .theta..sub.2 of the light beam, which in turn exerts significant influences to the devices and instruments used in combination with the optical system, to a great disadvantage. The variation or change in the wavelength of the light beam is ascribable to various causes such as dispersion of wavelength of the light emitted by a light source such as, for example, a semiconductor laser, change of the emitted wavelength in the course of time lapse, change in the ambient temperature, change in the light output energy and the like.